1. Field of the Invention
Generally, the present invention relates to the field of telecommunication devices for the deaf (TDDs) or text telephone yokes (TTYs). More particularly, the invention relates to modification of standard vocoder operation to enable reliable transport of TDD/TTY signals within a telecommunication system. The system may include wireless links.
2. Description of the Related Art
Many deaf or hearing-impaired people use communication terminals specifically constructed and designed to enable them to communicate over standard telephone lines. Such devices, referred to as telecommunication devices for the deaf (TDDs) or Text Telephone Yokes (TTYs), are collectively referred to as TTDs in this application. Typically, TTDs include a keyboard and a display connected to a telephone via a modem (modulator/demodulator). The modem is built into the TDD and is either directly connected to a telephone line or coupled by an acoustic coupler to a normal telephone handset. TDDs are capable of transmitting information over telephone lines by means of coded tones to other TDDs connected at opposite ends of the telephone line through another modem. These tones are referred to as low activity communications because the frequency and amplitude envelopes remain relatively constant.
The code and protocol that is in widespread conventional use for TDD communications is an idiosyncratic one. The code set, known as Baudot, and the communication protocol (TDD protocol) evolved historically at a time when many telecommunication devices for the deaf were based on mechanical or electromechanical devices rather than electronic devices. Accordingly, the TDD protocol was constructed for a set of constraints that no longer are relevant to present day devices. Those constraints work to create a code protocol and a telecommunication network of users and devices operating under that protocol, that is somewhat antiquated.
Traditionally, TDD communications are conducted at 50 Baud (45.5 Baud in some countries), representing a transfer of 6 characters per sec. Other protocols now available for TDD communications incorporate higher Baud rates, such as the ASCII (American Standard Code Information Interchange) and enhanced Baudot protocols. Regardless, a normal TDD communication character set consists of characters that are 5 bits long. These characters are analogous to a letter in an alphabet where each letter represents a word or idea. A character is grouped with overhead information bits prior to transfer, where each group of bits to be transferred has a duration or unit interval equal to 20 milliseconds. For example, under conventional TDD protocol, a group of bits to be transferred comprises 8 bits: a start bit (one source or zero bit), five bits representing the character, and at least one and xc2xd bits marking the stop point of the transfer group. This 20 milliseconds interval is also the frame length produced by a vocoder, discussed below, for transmitting one frame of information in a wireless telecommunications system.
Compared to modem telecommunication systems, TDD transmissions occur at a snail""s pace. A bigger problem is that TDD signals are substantially constant. These slow paced, monotone signals can create havoc in digital telecommunication systems that transmit higher activity signals at very high rates, and especially in telecommunication systems that include wireless links. One example of such a telecommunication system is a code division multiple access (CDMA) system having a large number of wireless subscriber units. Each subscriber unit has a transceiver and communicates within the system through satellite repeaters or terrestrial stations referred to as cells. Each cell includes a physical plant called a base station. A cell covers a limited geographic area and routes calls carried over subscriber units to and from the telecommunication network via a mobile switching center. When a subscriber moves into the geographic area of a new cell, the routing of that subscriber""s call may be eventually made through the new cell by a process called a xe2x80x9chandoff.xe2x80x9d
A subscriber unit, generically referred to as a cell phone, transmits a signal that is received by a base station. The signal is relayed to a mobile switching center that routes the signal to a public switched telephone network (PSTN) including telephone lines or other subscriber units. Similarly, a signal may be transmitted from the PSTN to a subscriber unit via a base station and a mobile switching center.
The interface between the subscriber unit and the base station is referred to as the air interface. The telecommunications industry association (TIA) has provided a standard for CDMA call processing on the air interface entitled xe2x80x9cIS-95 Mobile Stationxe2x80x94Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System.xe2x80x9d Addendum to IS-95 are provided as Telecommunications Service Bulletins (TSB). The standard IS-95+TSB74 includes provisions for service negotiation on the air interface and is incorporated herein by reference.
Service negotiation is critical to successfully transmit any communication, especially a low activity TDD communication, over a digital telecommunication system. One problem with modern systems, including the one described above, is that a vocoderxe2x80x94a device used in the system to encode a voice or TDD analog signal into a digital signal, and to decode a digital signal into a voice or TDD analog signalxe2x80x94has difficulty in handling the substantially monotone signal and slow speed dictated by the TDD protocol. In current systems, a low activity communication signal such as a TDD communication would probably be treated by the vocoder as background noise or signal interference and be disregarded.
What is needed is an invention that can easily be integrated into existing communication systems and that does not require an increase in transmission power to reduce frame error rates. The invention should be able to sense when a low activity communication is received, or notice should be sent to the system, and be able to reconstruct frames containing errors by reviewing adjacent frames. Alternatively, the invention should be capable of reducing frame error rates by invoking a protocol to be used by the vocoders during transmission of the low activity communication signal.
The invention should be compatible with wireless telecommunication modulation systems, such as CDMA systems, servicing large numbers of system users. A more robust discussion of CDMA systems and techniques used in multiple access communication systems may be found in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein. Further, the invention should also be compatible with other modulation systems and techniques used in other types of communication systems, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AMPS) schemes.
Broadly, the present invention involves the modulation of a low activity communication by a telecommunication system using encoded signals. More particularly, the invention concerns a method, article of manufacture, and apparatus that uses specialized encoding, decoding, or both, on a low activity communication signal to minimize a transmitted signal""s frame erasure rate without increasing transmission power. The invention also provides for decoding a low activity signal by looking at xe2x80x9csoft bitsxe2x80x9d contained in erred frames, or in frames adjacent to an erred frame, in an attempt to determine the content of the original frame.
Certain disclosed embodiments of the invention provide unique decoding methods for a TDD signal that was encoded using standard encoding protocol. In one embodiment, the decoder may compare a frame containing transmission errors (erred frame) with a vocoded frame from a known TDD signal and determine the most likely vocoded frame that was transmitted. In another embodiment, the decoder may review adjacent frames to determine the most likely vocoded frame that was transmitted but received in error. And although a TDD communication is discussed throughout this application, it should be understood that any slow or low activity communication may be transmitted using this invention.
Another embodiment of the invention provides for decoding as discussed above but invokes vocoder parameters that are different from standard vocoder parameters. When a TDD signal is received, the encoder switches to xe2x80x9cBaudot encoding mode,xe2x80x9d notices the decoder of the protocol change, and uses channel coding redundancy to further improve the decoder""s chances of determining the correct TDD signal sent even if it is contained in a bad frame. This version of the invention replaces standard vocoder parameters with vocoder xe2x80x9csignaturesxe2x80x9d that are better spaced apart, thus making it easier to distinguish between tones.
Yet another version of the invention provides for encoding a TDD signal in vocoder frames using redundancy, but doing the encoding across numerous vocoder frames. The information is interleaved across xe2x80x9cNxe2x80x9d frames so that if a frame is lost, the decoder can extract necessary information from adjacent frames to determine the content of the lost frame.
The invention provides its users with numerous advantages. One advantage is that a TDD message can be transmitted using a digital transmission medium having wireless links. Yet another advantage is that a TDD device can be connected to a mobile device or subscriber""s unit, such as a digital cellular telephone, connected to the telecommunications system by a wireless link. The invention also provides a number of other advantages and benefits that should become even more apparent after reviewing the following detailed descriptions of the invention.